Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2–C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds are subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1–C4 alkanols, especially methanol and ethanol; C2–C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a molecular sieve catalyst composition. Molecular sieves are porous solids having pores of different sizes, such as zeolites or zeolite-type molecular sieves, carbons and oxides. The most commercially useful molecular sieves for the petroleum and petrochemical industries are zeolites, for example aluminosilicate molecular sieves. Zeolites in general have a one-, two- or three-dimensional crystalline pore structure having uniformly sized pores of molecular dimensions that selectively adsorb molecules that can enter the pores, and exclude those molecules that are too large.
There are many different types of molecular sieve well known to convert a feedstock, especially an oxygenate containing feedstock, into one or more olefin(s). For example, U.S. Pat. No. 5,367,100 describes the use of the zeolite, ZSM-5, to convert methanol into olefin(s); U.S. Pat. No. 4,062,905 discusses the conversion of methanol and other oxygenates to ethylene and propylene using crystalline aluminosilicate zeolites, for example Zeolite T, ZK5, erionite and chabazite; U.S. Pat. No. 4,079,095 describes the use of ZSM-34 to convert methanol to hydrocarbon products such as ethylene and propylene; and U.S. Pat. No. 4,310,440 describes producing light olefin(s) from an alcohol using a crystalline aluminophosphate, often designated AlPO4.
Some of the most useful molecular sieves for converting methanol to olefin(s) are silicoaluminophosphate molecular sieves. Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystalline framework structure of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units. SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO molecular sieves are generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formulation into a SAPO catalyst, and its use in converting a hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, are disclosed in U.S. Pat. Nos. 4,499,327, 4,677,242, 4,677,243, 4,873,390, 5,095,163, 5,714,662 and 6,166,282, all of which are herein fully incorporated by reference.
Conversion of oxygenates to olefins generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Over-accumulation of these carbonaceous deposits will inhibit the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on the molecular sieve catalyst, the oxygenate to olefin process generally incorporates a second step comprising catalyst regeneration. During regeneration, the coke is removed from the catalyst, typically by combustion with oxygen, which at least partially restores the catalytic activity of the catalyst. The regenerated catalyst then may be reused to catalyze the conversion of oxygenates to olefins.
For example, SAPO-34 is known to be a selective molecular sieve catalyst in the conversion of methanol to ethylene and propylene. However, its excellent selectivity to ethylene and propylene (maximum selectivity about 40–43 wt % each) requires the formation of a carbon pool as carbonaceous material is being deposited on the catalyst. With fresh catalyst, ethylene and propylene selectivities are about 20–24 wt % and about 32–36 wt % respectively rising to their maximum values with time as more carbonaceous material is being deposited. However, catalyst activity drops off rapidly when the carbonaceous material is greater than about 10 wt % (based on SAPO-34 molecular sieve content). Fixed bed operation is not practical since catalyst life under a reasonable space velocity (at least 3 w/w/hr) is less than 2.5 hours. For this reason most current proposals for converting oxygenates to olefins employ a fluidized bed reactor in which the catalyst is continuously circulated between a riser reactor and a regenerator.
It would, however, be desirable to provide an improved molecular sieve catalyst composition and process which would enable a fixed bed reactor system to be employed in an economically viable manner in the conversion of oxygenates, such as methanol, to olefins.
U.S. Pat. No. 4,873,390 to Lewis et al., incorporated herein by reference, teaches conversion of a feedstock, e.g., alcohols, to a product containing light olefins over a silicoaluminophosphate catalyst having pores with a diameter of less than 5 Angstroms, wherein a carbonaceous deposit material is formed on the catalyst. The catalyst is treated to form a partially regenerated catalyst having from 2 to 30 wt. % of the carbonaceous deposit material. The catalyst is preferably in a fluidized state and is continuously transported between the reaction zone and the regeneration zone.
U.S. Pat. No. 6,023,005 to Lattner et al., incorporated herein by reference, discloses a method of producing ethylene and propylene by catalytic conversion of oxygenate in a fluidized bed reaction process which utilizes catalyst regeneration. The process maintains a portion of desired carbonaceous deposits on the catalyst by removing only a portion of the total reaction volume of coked molecular sieve catalyst and regenerating only that portion of catalyst, which is then mixed back with the unregenerated remainder of catalyst. The resulting catalyst mixture contains 2–30 wt % carbonaceous deposits.
In our co-pending U.S. Patent Publication No. 2003/0176752 published Sep. 18, 2003, incorporated herein by reference, there is described a catalyst composition that exhibits enhanced lifetime when used in the conversion of oxygenates to olefins and which comprises a molecular sieve and at least one metal oxide having an uptake of carbon dioxide at 100° C. of at least 0.03 mg/m2 of the metal oxide. The metal oxide is selected from an oxide of Group 4 of the Periodic Table of Elements, either alone or in combination with an oxide selected from Group 2 of the Periodic Table of Elements and/or an oxide selected from Group 3 of the Periodic Table of Elements, including the Lanthanide series of elements and the Actinide series of elements.
In our co-pending U.S. Patent Publication No. 2003/0176753 published Sep. 18, 2003, incorporated herein by reference, there is described a catalyst composition that exhibits enhanced lifetime when used in the conversion of oxygenates to olefins and which comprises a molecular sieve and at least one metal oxide having an uptake of carbon dioxide at 100° C. of at least 0.03 mg/m2 of the metal oxide. The metal oxide is selected from an oxide of a metal from Group 3 of the Periodic Table of Elements, the Lanthanide series of elements and the Actinide series of elements.
U.S. Pat. No. 4,789,528 to Owen et al., incorporated herein by reference, teaches a multi-reactor system for the conversion of light olefins to gasoline and/or distillate in which at least three reactors are sequentially rotated between a regeneration mode and at least two serially connected reaction modes respectively. In the reaction modes, the reactor having relatively less fresh catalyst receives feed from a feed source first and the reactor having the fresher catalyst receives the feed last.